1. Field of the Invention
The present invention relates to a process for producing a muscone and an intermediate of the muscone and, more particularly, to a process capable of producing an intended muscone in a short process at a high yield and also relates to a novel intermediate useful for producing the muscone.
2. Description of the Related Art
Recently, the orientation of people toward natural products is increased, and with regard to highly preferred perfumes allowing the imagination specific to nature, it has been strongly desired to develop fragrances derived from natural compounds or fragrances that are the same as or similar to natural compounds in view of safety.
A muscone that is one of such fragrances and is represented by the following formula (6) is a compound that is a major fragrance component of natural musk. It is contained in an amount of 0.5 to 2.0% in natural musk and was discovered by Walbaum in 1906. It has a chemical structure determined by Ruzicka in 1926. A natural muscone is (−)-(R)-3-methylcyclopentadecanone while a commercial product is a synthetic product and a d1-isomer.

When the fragrance of a (−)-(R)-isomer of a muscone is compared with that of a (+)-(S)-isomer of a muscone, the (R)-isomer has a diffusive musk fragrance (threshold value: 3 ppm) whereas the (S)-isomer has a chemically non-expansive, poor and weak musk fragrance (threshold value: 10 ppm), with the result that as to the intensity of fragrance, it is known that the (R)-isomer has an intensity of fragrance three times stronger than that of the (S)-isomer (Motoichi Indo, “Synthetic Perfume, Chemistry and Product Information”, The Chemical Daily Co., Ltd., published on Mar. 3, 1996, pp. 492-497; and “Latest Technologies of Synthetic Perfume”, CMC Publishing Co., Ltd., published in 1982, pp. 72-90).
In view of this situation, many studies have been made as to a method of producing a muscone and particularly, a (−)-(R)-muscone and the results of these studies have been reported. Among these methods, there are some methods that have been reported for producing a (−)-(R)-muscone using optically active citronellal as an asymmetric source. For example, a method has been reported in which (1) a hydroxyl group of a long-chain secondary alcohol synthesized from optically active citronellal and 9-decenyl Grignard reagent is protected with a silyl group, (2) the both-terminal dialdehyde group generated by ozonolysis is converted into a both-terminal diolefin by a methyl Wittig reaction, (3) the silyloxy group is deprotected, (4) the hydroxyl group is converted into a ketone by Jones oxidation, followed by (5) metathesis cyclization and then (6) the double bond is hydrogenated to obtain an optically active muscone (J. Chem. Soc., Perkin., 1, 2253 (1998) which is incorporated herein by reference).
This production method, however, involves using expensive 9-decenyl Grignard reagent, and requires many steps including a step of protecting and deprotecting a hydroxyl group, two oxidizing steps for ozonolysis and Jones oxidation and a Witting reaction step using a butyllithium reagent and, therefore, has the drawback that the process is too long. It must be said that it is difficult to adopt this method for industrial production of a (−)-(R)-muscone.
Also, a method has been reported in which a shorter process is attained wherein a long-chain secondary alcohol synthesized from optically active citronellal and a 9-decenyl Grignard reagent is subjected to metathesis cyclization, the obtained macrocyclic alcohol is subjected to a dehydrogenation reaction to make a corresponding ketone and then the double bond is hydrogenated to obtain an optically active muscone (J. Am. Chem. Soc., 123, 11312 (2001))
However, in this method, it is likewise necessary to use an expensive 9-decenyl Grignard reagent and therefore it cannot be said that this method is satisfactory.